The present invention relates to non-impact printers and, more particularly, to laser printers.
As a result of both improvements in technology and reductions in price, non-impact printers have grown in popularity over the recent years. For example, it is now possible for a small business, or even an individual, to purchase a non-impact printer which, in conjunction with a word processing system, can produce high-quality announcements, letters, reports, etc., that were previously only available from other sources such as an outside printing shop. Currently, non-impact printers cover a range of technologies, e.g., electrophotographic, metal drum, and ink jet printers. However, the best known of these are the electrophotographic printers, i.e., the laser and light-emitting-diode (LED) printers.
Both the laser- and LED-based printers have the same goal, which is to scan an image with light onto a photosensitive surface. The photosensitive surface is usually a photo-drum which is electrically charged and draws toner to the charged areas which is then applied to a page and fused by heat.
A laser printer is built around a "printer engine" which includes a semiconductor laser and a rotating scanning mechanism (e.g., a rotating mirror). As the scanning mechanism rotates, it deflects a light beam from the semiconductor laser across the photo-drum thereby scanning a line of images onto the photo-drum. Unfortunately, the laser printer engine is subject to a number of limitations as a result of the scanning mechanism. Specifically, the presence of the scanning mechanism means that the laser printer will be sensitive to vibration. It also has an effect on the printer's reliability (due to the moving parts of the scanning mechanism), and, more importantly, results in a basic limitation on the printing speed, the latter being limited by the rotation speed of the scanning mechanism.
As a result, those in the art have addressed these limitations by offering various alternative designs for laser-based printers. For example, U.S. Pat. No. 4,445,125 issued Apr. 24, 1984 to Scifres et al. discloses a laser array and a scanning mechanism which can illuminate a plurality of lines on the photosensitive medium; U.S. Pat. No. 4,786,918 issued November 1988 to Thornton et al. discloses a plurality of lasers, used as a single light source, which uses a total internal reflection (TIR) line modulator for spacial modulation and a plurality of focusing lenses; and U.S. Pat. No. 4,900,130 issued Feb. 13, 1990 to D. Haas discloses a plurality of light sources and a scanning mechanism which is further modified to move transversely in respect to the photo-drum.
In contradistinction, an LED-based printer employs a stationary panel, or array, of light sources in which a separate diode is used as a source of light for each dot that comprises an image. The stationary panel of LEDs is as wide as the width of the paper, so that there is a 1:1 relationship between the size of the dots that comprise the image and the discrete LEDs which comprise the panel. In addition the stationary panel must be mounted close to the photosensitive medium, or photo-drum, due to power considerations inherent in the use of LEDs as a light source. As a result of the use of such a stationary panel, an LED-based printer can offer increased printing speeds compared to a laser-based printer since there is no time required to scan a light source across the photo-drum, and may be more reliable than a laser printer due to the lack of the rotating scanning mechanism. Unfortunately, problems may exist in the construction of the stationary panel due to its size (e.g., the width of a panel to cover a letter-size paper is eight inches). In addition, LEDs as a light source offer less resolution than is possible with a laser-based printer.
Notwithstanding some of these limitations, LED-based printers are currently becoming more popular in the marketplace than laser-based printers due to the increased printing speeds mentioned above. This has resulted in electrophotographic printer manufacturers beginning to focus more efforts on the further development of LED-based printers.